Field of the Invention
The present invention relates to a method for hot forming, in particular for press hardening, a component, in particular for a vehicle.
Description of Related Art
On account of their high characteristic strength values, hot formed or press hardened components, respectively, are inter alia employed in vehicle construction as safety-relevant components. As the contact pressure per unit area increases, the hardness in the material increases on account of higher cooling rates. Excessive hardness in the material implies a higher probability of a fracture arising in the component which has been manufactured from the material. There is a requirement for a method which is suitable in mass production for hot forming, in particular for press hardening, components which in a predefined local region have a reduced hardness and thus a lower probability of brittle fractures arising.
Some press hardening methods for producing a locally differentiated material hardness, such as partial press hardening, for example, are already known in the prior art. In the case of tailored tempering, for example, the pressing tools are only partially heated during hot forming. By way of targeted temperature control, a mixed microstructure with reduced strengths is produced in a localized manner here. However, here a martensitic formation of the material is suppressed in a targeted manner by way of reduced cooling rates. In a localized manner, tensile strengths of between 500 and 950 MPa are set. The resulting Vickers hardness is in the range of 170 HV to 320 HV, which may compromise the use of the component as a lightweight element.
Likewise, there are attempts at modifying the alloy composition of the tool steel such that the tool steel has a locally reduced heat conductivity, on account of which the cooling rate of the component to be pressed drops in a localized manner and thus a lower hardness is achieved in a localized manner. In such alloy concepts with locally reduced heat conductivities according to the prior art, an operating temperature has to be first set in order to obtain a reduced cooling rate of the material to be hardened. From the point of view of production, this does not represent a reliable process. Therefore, attempts including alloy compositions of the tool steel which have been especially modified for press hardening to date have not been implemented in large-scale production environments.